Flame detectors or flame scanners are devices which are used to determine the state of burners in industrial and utility furnaces. Such furnaces can be, for example, steam boilers, water heaters, or gas-, oil- or coal-fired furnaces. Flame detectors monitor one or several flames inside a furnace. In a conventional flame detector phototubes or photodiodes are used to detect the total light intensity of the flame which is received via some focussing optics.
FIG. 1 depicts a state-of-the-art flame detector 1 that is mounted on a burner 2. The burner 2 has a tapered burner nozzle 3 with vanes 4 on its outside. Fuel (black arrow) and air (white arrow) are led though or alongside the burner 2 into a non-depicted furnace. One version of a known flame detector 1 consists of a carrier tube 5 and a photo element in form of a photodiode 6 that is mounted at a front end of the carrier tube 5. The front end of the carrier tube 5 is located at the aperture of the burner nozzle 3 which is directed toward the inside of the furnace i.e. toward the combustion chamber. The tube also carries cooling air to the photodiode 6 and includes cables for the power supply of the photo element 6 and for transmitting the data signals recorded by the photodiode 6.
Another known flame detector 1 comprises at the front end of the tube 5 instead of a photodiode a lens 7 that focuses the light of the flame inside the furnace onto a fiber optic cable that is located inside the carrier tube 5. In this case, a photo element is located at the rear end of the carrier tube 5 in a separate casing. The photo element receives the light from the flame via other lens 7 and the fiber optic cable. The casing of the photo element is mounted at the outside of the furnace where ambient temperatures prevail. A signal conditioning unit is provided inside the casing of the photo element. From the signal conditioning unit the data signals are transmitted via wires to flame detection modules and further to a burner/boiler management system (BMS).
Phototubes comprising a tube and a photo element, in particular a photodiode, or a lens mounted at the front end of the tube may, however, require a precise line-of-sight for flame evaluation.
From patent application US 2005/0266363 A1 it is also known to collect and transmit light from several flames by use of optical fibers, to insect the collected light by a video camera vision system at the other end of the optical fibers and to transmit the “life” images of the glows of the flames as well as the “on/off” status of the burners to a control room.
From patent application DE 196 32 174 A1 a device for measuring the temperature of a flame, in particular a flame inside a combustion chamber of a gas turbine, is known that comprises an optical sensor fiber that is directed toward the flame and connected to a spectrograph for analyzing the spectral composition of the flame image.
From patent documents EP 0 616 200 B1 and US 2001/0014436 A1 it is known to employ cameras or other scanning devices for monitoring flames in furnaces. According to the patent document U.S. Pat. No. 5,249,954 A a camera is mounted at the rear end of a sight tube which extends through a windbox into the furnace. Hence, the camera views the flame through the sight tube. Soot from the flame may, however, cover the front end of the sight tube or an observation window behind which the camera is positioned which may lead to degradation of the flame detection capability of the camera.
Feasible flame detectors have, furthermore, to be constructed such that they can withstand high temperatures, flame temperatures usually being around 1500° C. and the wall temperatures of the furnace walls usually being around several hundred degrees Celsius.